所有的协程都是在上下文中运行,而协程上下文包含一个协程调度器.
协程调度器确定了相关的协程在哪个线程或哪些线程上执行,协程调度器可以将协程限制在一个特定的线程执行,或将它分派到一个线程池,亦或是让它不受限地运行.
协程调度器源码:
public actual object Dispatchers {
/**
* The default [CoroutineDispatcher] that is used by all standard builders like
* [launch][CoroutineScope.launch], [async][CoroutineScope.async], etc
* if no dispatcher nor any other [ContinuationInterceptor] is specified in their context.
*
* It is backed by a shared pool of threads on JVM. By default, the maximal level of parallelism used
* by this dispatcher is equal to the number of CPU cores, but is at least two.
* Level of parallelism X guarantees that no more than X tasks can be executed in this dispatcher in parallel.
*/
@JvmStatic
public actual val Default: CoroutineDispatcher = createDefaultDispatcher()
/**
* A coroutine dispatcher that is confined to the Main thread operating with UI objects.
* This dispatcher can be used either directly or via [MainScope] factory.
* Usually such dispatcher is single-threaded.
*
* Access to this property may throw [IllegalStateException] if no main thread dispatchers are present in the classpath.
*
* Depending on platform and classpath it can be mapped to different dispatchers:
* - On JS and Native it is equivalent of [Default] dispatcher.
* - On JVM it is either Android main thread dispatcher, JavaFx or Swing EDT dispatcher. It is chosen by
* [`ServiceLoader`](https://docs.oracle.com/javase/8/docs/api/java/util/ServiceLoader.html).
*
* In order to work with `Main` dispatcher, the following artifacts should be added to project runtime dependencies:
* - `kotlinx-coroutines-android` for Android Main thread dispatcher
* - `kotlinx-coroutines-javafx` for JavaFx Application thread dispatcher
* - `kotlinx-coroutines-swing` for Swing EDT dispatcher
*
* In order to set a custom `Main` dispatcher for testing purposes, add the `kotlinx-coroutines-test` artifact to
* project test dependencies.
*
* Implementation note: [MainCoroutineDispatcher.immediate] is not supported on Native and JS platforms.
*/
@JvmStatic
public actual val Main: MainCoroutineDispatcher get() = MainDispatcherLoader.dispatcher
/**
* A coroutine dispatcher that is not confined to any specific thread.
* It executes initial continuation of the coroutine in the current call-frame
* and lets the coroutine resume in whatever thread that is used by the corresponding suspending function, without
* mandating any specific threading policy. Nested coroutines launched in this dispatcher form an event-loop to avoid
* stack overflows.
*
* ### Event loop
* Event loop semantics is a purely internal concept and have no guarantees on the order of execution
* except that all queued coroutines will be executed on the current thread in the lexical scope of the outermost
* unconfined coroutine.
*
* For example, the following code:
* ```
* withContext(Dispatchers.Unconfined) {
* println(1)
* withContext(Dispatchers.Unconfined) { // Nested unconfined
* println(2)
* }
* println(3)
* }
* println("Done")
* ```
* Can print both "1 2 3" and "1 3 2", this is an implementation detail that can be changed.
* But it is guaranteed that "Done" will be printed only when both `withContext` are completed.
*
*
* Note that if you need your coroutine to be confined to a particular thread or a thread-pool after resumption,
* but still want to execute it in the current call-frame until its first suspension, then you can use
* an optional [CoroutineStart] parameter in coroutine builders like
* [launch][CoroutineScope.launch] and [async][CoroutineScope.async] setting it to the
* the value of [CoroutineStart.UNDISPATCHED].
*/
@JvmStatic
public actual val Unconfined: CoroutineDispatcher = kotlinx.coroutines.Unconfined
/**
* The [CoroutineDispatcher] that is designed for offloading blocking IO tasks to a shared pool of threads.
*
* Additional threads in this pool are created and are shutdown on demand.
* The number of threads used by this dispatcher is limited by the value of
* "`kotlinx.coroutines.io.parallelism`" ([IO_PARALLELISM_PROPERTY_NAME]) system property.
* It defaults to the limit of 64 threads or the number of cores (whichever is larger).
*
* Moreover, the maximum configurable number of threads is capped by the
* `kotlinx.coroutines.scheduler.max.pool.size` system property.
* If you need a higher number of parallel threads,
* you should use a custom dispatcher backed by your own thread pool.
*
* This dispatcher shares threads with a [Default][Dispatchers.Default] dispatcher, so using
* `withContext(Dispatchers.IO) { ... }` does not lead to an actual switching to another thread —
* typically execution continues in the same thread.
*/
@JvmStatic
public val IO: CoroutineDispatcher = DefaultScheduler.IO
}
从源码可以得知
Main 是运行在主线程的及在Android中进行UI绘制的线程.一般用于处理UI绘制及一些轻量级任务
Default 是运行在非主线程的,一般用于CPU密集型计算型任务.如数组排序,JSON解析,异常判断等
IO 是运行在非主线程的,一般用于磁盘操作和网络IO操作任务.如数据库操作,文件读写,网络请求等
Unconfined 未定义的线程,是一个特殊的调度器且似乎也运行在 main 线程中,但实际上它是一种不同的机制.协程调度器在调用它的线程启动了一个协程,但它仅仅只是运行到第一个挂起点.挂起后,它恢复线程中的协程,而这完全由被调用的挂起函数来决定.非受限的调度器非常适用于执行不消耗 CPU 时间的任务,以及不更新局限于特定线程的任何共享数据（如UI）的协程
